ABSTRACT Functional outcomes vary following total knee replacement (TKR), the most common and costly inpatient procedure. Although post-surgical physical therapy (PT) is routinely prescribed in perioperative care, the actual PT content delivered to patients varies widely, and no consensus exists for the type, amount or intensity of PT following TKR. In 2016, Medicare will implement mandatory bundled payment for TKR in 67 municipalities across the U.S. making the surgeon and hospital responsible for all post-TKR costs, including physical therapy. These financial incentives may alter post-TKR physical therapy services without a clear understanding of the specific PT components that improve patient outcomes. Our recent retrospective study of PT care from 50 therapy sites in one geographic region documented a significant association between exercise type and progression and greater functional outcomes. Clinical trials demonstrate that muscle strengthening and functional exercises improve functional outcomes following TKR but the generalizability of these studies is limited. In contrast, practice-based research allows analysis of the variability of PT practice in a large patient sample and assessment of the association between PT care, patient participation, and functional outcomes. We propose a PT cohort study of TKR patients embedded within a national TKR cohort (FORCE-TJR) to collect specific details of all post-TKR physical therapy care from 360 patients treated by approximately 60 PTs in three geographical areas. Baseline patient attributes and outcome measures are routinely collected in the FORCE data. These data will be expanded to include the ambulatory PT intervention details as well as the patients? participation in home exercise and physical activity. A web-based data capture system for use by physical therapists and patients will collect details of amount, type and intensity of physical therapy exercise and amount of patient participation in the rehabilitation process. The study aims are to: (1) Quantify the variation in outpatient PT components and progression in intensity, and the patient?s in home participation in prescribed rehabilitation activities following TKR in a multi-center prospective sample; (2) Identify PT components associated with optimal patient-reported functional outcomes at 6 and 12 months and knee performance measures at 6 months after TKR; and (3) Design and deploy a web-based decision support tool to predict functional outcome after TKR based on these analyses, to include patient pre-operative factors, levels of patient participation, and PT components to reinforce best practice PT and patient in-home participation. We hypothesize that a set of potentially modifiable PT practice components, such as a higher number of closed chain exercises and progressions in exercise intensity, will be positively associated with better gain in functional and performance outcomes after adjusting for patient sociodemographic and comorbidity factors and that patient adherence to daily at-home participation in physical activity will modify the effects of the PT components on knee performance and function at 6 months. Statistical modeling will examine the role PT timing, content and intensity and patient demographic and clinical factors including rehabilitation participation play in explaining the variations in functional outcomes following TKR. Predictive models of long term functional outcomes based on both PT and patient factors will identify modifiable factors of the PT intervention that contribute to optimal functional outcomes. The data and knowledge generated by this study will inform the design and content of a prospective randomized trial of best practice PT after TKR among patients at risk for poor functional gain. Moreover, the knowledge is timely and essential as policy makers and clinicians work to control the rising societal cost of total knee replacement surgery, while assuring optimal patient outcomes.